Question

Why were dobereiner triad's rejected?

Answer 1

Dobereiner's Triad:

• Dobereiner stated that the arithmetic mean or the average of the atomic masses of the first and third elements in a triad would be approximately equal to the atomic mass of the second element in that triad.
• He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties.
• For example:

Calcium, barium, and strontium formed one of Dobereiner’s triads:

Triad	Atomic Masses
Calcium	40.1
Strontium	87.6
Barium	137.3

The average of the masses of barium and calcium equals $\left(\frac{40.1+137.3}{2}\right)$= 88.7 which is near 87.6 so they formed a triad and the properties of these three elements were also similar.

Limitations of Dobereiner triad:

• The main Limitation of Dobereiner's classification of elements was that he failed to arrange all the then-known elements in the form of triads of elements having similar chemical properties.
• For very low mass or for very high mass elements, the law was not applicable. For an example of F, Cl, Br, the atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.
• Newly discovered elements did not fit into the triads.
• There were only 5 Dobereiner’s triads identified.
• Several known elements did not fit into any of the triads.

So due to these limitations, Dobereiner triads were rejected.